In optical communication systems that use polarization multiplexing (POLMUX), two optical signals are polarized orthogonally with respect to each other before transmitted in an optical channel at a certain wavelength. This doubles the bandwidth of the channel. To realize a high-speed transmission over 100 Gbps, POLMUX and dense-wavelength-division multiplexing (DWDM) are demanded.
However, the constantly alternating bit sequences of the POLMUX signal cause the resulting polarization to change over time. This depolarization effect results in cross-polarization modulation (XPolM), which is essentially noise. The XPolM impairment is induced by inter-channel fiber nonlinearity particularly for DWDM optical communication systems. Due to the XPolM, the dual-polarized (DP) signals, which originally were polarized orthogonally, are no longer so, and cannot be separated at a receiver.
XPolM causes significant performance degradation in bit error rate or in quality factor for DP coherent optical systems. The induced polarization crosstalk has a low-pass response because of chromatic dispersion, resulting in a short coherence time of tens of symbols. Such a fast time-varying polarization crosstalk is hard to track for the current optical communication systems.
The prior art technique, which compensates time-varying XPolM effect, is based on direct cancellation followed by a tentative decision by introducing a simplified XPolM crosstalk model as shown in FIG. 1. For the prior art scheme, hard decisions 110 are first performed to estimate the transmitted signals, assuming no XPolM. Then, the polarization crosstalk factors are estimated 120 by averaging over a number of symbols 130 to reduce the impact of noise. Given these estimated crosstalk factors, the interference is canceled 140.
This cancellation method is simple and it is inaccurate especially at a low signal-to-noise ratio (SNR) due to errors being generated in the initial estimation of the transmitted signal using hard decisions of the received signal. Moreover, this technique is not applicable in the presence of another non-linear effect including cross-phase modulation (XPM) because the simplified XPolM crosstalk model is not valid.